1. Field of the Invention
This invention relates generally to wireless communication systems that includes security arrangements for preventing any unauthorized use and has particular applicability to cordless telephone systems.
2. Description of the Related Art
The cordless telephone has become a popular consumer good. The cordless telephone allows a user to untether herself from a wired connection to her local telephone line. Typically a cordless telephone is comprised of two units: a base unit and a handset, both of which are radio frequency (xe2x80x9crfxe2x80x9d) transceivers. The base unit connects to the public switched telephone network, typically using a standard RJ-11 connector. The base unit provides a wireless connection or a wireless communication link through the handset. The handset is capable of receiving and transmitting signals over a wireless link to the base unit. The base unit typically includes an antenna, a transmitter and a receiver. The handset typically includes a speaker, a microphone, an antenna, a transmitter and a receiver.
In operation, to place a telephone call from a cordless telephone handset, the handset is enabled causing a control signal to be generated at the handset and transmitted to the base unit. The base unit receives and detects the control signal and in response thereto seizes the telephone line. Dialing signals and audio signals from the handset are transmitted (using various known transmission schemes and formats) to the base unit which then transmits them over the telephone line. The base unit receives audio information over the telephone line and then transmits that information to the handset.
With the ever increasing use of cordless telephones and the limited frequency band available for transmission between handsets and base stations, many users are assigned the same frequencies for transmitting and receiving. People cognizant of this fact have used handsets to place unauthorized telephone calls through the base stations of other users. With the range of communication between a base unit and a handset typically extending at least a few hundred feet, it is quite feasible that a person may travel around slowly in an automobile with a handset unit turned on until she receives a dial tone and then places a call over another person""s telephone lines.
In order to prevent the placing of unauthorized telephone calls, various systems have been developed to attempt to ensure that unauthorized telephone calls are not placed. One such system involves a control code that is determined by switches manually preset in both the handset and the base unit. Only after receipt of a signal with this control code from the handset and the favorable comparison thereof with the control code set at the base unit, does the base unit allow the telephone line to be seized to place the call from the handset. The possibility of mismatching the switch settings in the handset and the base unit is high, however, and the number of switches is therefore usually kept to a small number. Unfortunately this increases the opportunity for an unauthorized user to correctly determine the control code. Additionally, varying the switch setting between units during production increases production costs.
Some cordless telephone systems are designed to operate such that the handset and base station never come into physical contact. However, to achieve a secure wireless link, both the handset and the base unit need to know the same control or security code. If the system is designed such that this same security code is used by all base units in production, then unauthorized users can easily communicate with a base station and make unauthorized calls. Again, programming a unique security code into each handset and base unit pair at the factory can add significantly to production costs. It also precludes multi-handset unit systems and precludes the use of additional handsets with the base station.
In another security system for use with cordless telephones, the radio frequency link between the base unit and the handset is used to set the security code. Sometimes this is done at a lower power transmission to help reduce the chance of an unauthorized user receiving this xe2x80x9cset upxe2x80x9d transmission. However, even in the lowest available power setting, most base units still transmit in a range that far exceeds the buildings in which they are used and thereby potentially allows unauthorized users to intercept this communication.
In still another security system for use with cordless telephones, to prevent an unauthorized handset from obtaining dial tone from a base unit, a predetermined security code stored in the base unit is transferred to the handset while the handset is located in a mating cradle in the base unit. The battery that allows for operation of the handset while remote from the base unit is normally charged when the handset is placed in this mating cradle. The direct current charging path established for charging of the battery also includes transfer circuits in the base unit and the handset to respectively transmit and receive the security code. Such a system is described in U.S. Pat. No. 4,736,404, issued Apr. 5, 1988.
All of the foregoing described systems have addressed the problem of preventing unauthorized users from obtaining a dial tone and placing a call through another base unit. Another problem exists in the area of cordless telephones, and that is the problem of eavesdropping on the transmission between the base unit and the handset. Therefore, it would also be desirable to have the radio frequency transmissions between the base unit and handset be made more secure.
The invention is generally directed to methods and apparatus for establishing secure wireless links between a handset and a base station. One embodiment of the invention finds particular application to cordless telephone systems.
One aspect of the invention encompasses a method of generating a secure wireless link between a handset and a base station. The method includes initiating a linking procedure, generating a security code, transmitting sound based on the security code at the base station, receiving the sound at the handset, and then establishing a radio frequency link between the handset and the base station utilizing the security code.
In another aspect of the invention, a cordless telephone system capable of generating a secure wireless link includes both a handset and a base station. The handset includes a control circuit, a sound receiver, an rf transmitter, an rf receiver and a keypad, all coupled to the control circuit. The base station includes a control circuit, a code generation circuit coupled to the control circuit, a sound transmitter coupled to the control circuit for transmitting a code generated by the code generation circuit, and an rf transmitter and receiver coupled to the control circuit.
In a further aspect of the invention, the code generation circuit includes a pseudo random number generator.